Magnetospheric Balance of Solar Wind Dynamic Pressure

Abstract:

The magnetopause is the boundary established by pressure balance between the solar wind flow in the magnetosheath and the magnetosphere. Generally this pressure balance is represented to be between the solar wind the dynamic pressure and the magnetic pressure of Earth’s dipole field. The plasma actually in contact with the magnetosphere is the slowed, compressed, and heated solar wind downstream of the shock. The force exerted on the magnetosheath plasma is the JxB force produced by the Chapman-Ferraro current that flows on the magnetopause. Under typical solar wind conditions of relatively high magnetosonic Mach number flow (> 6), this simple picture is a reasonable description of the situation. However, under conditions of low solar wind magnetosonic Mach number flow (~2) the force on the solar wind plasma is not exerted at the magnetopause and must be exerted at the bow shock by currents that connect to the Region 1 currents. In this paper we present observations from two magnetopause crossings observed by the THEMIS spacecraft to compare and contrast the force balance with the solar wind for two situations with two very different solar wind magnetosonic Mach numbers.